As shown in FIG. 7 and FIG. 8, a loose-tube type optical fiber cable 101 is conventionally configured by bundling plural loose tubes 103 around a center tension member 105 with SZ twisting. A thin film 107 is sheathed around the bundled loose tubes 103. Further, a PE sheath 109 is provided around the thin film 107 as an outer sheath. Such a configuration of the loose-tube type optical fiber cable 101 is most commonly-used.
In addition, as shown in FIG. 9, the loose tube 103 is configured by accommodating plural optical fibers 111 within a tube (made of thermoplastic resin such as PBT resin) 113. Gel (jelly: filler: waterproof material) is filled within the tube 113. For example, an outer diameter of the tube is made 2.2 mm and an inner diameter thereof is made 1.4 mm, and the twelve optical fibers 111 are accommodated therewithin.
As shown in FIG. 10, in a conventional extrusion-molding apparatus for a loose tube, a needle 121, a tip 123 and a die 125 are to be concentrically arranged within a tube extrusion-molded in a extrusion head 119. The needle 121 feeds out the optical fibers 111 and the gel 115 to be filled around the optical fibers 111. The tip 123 and the die 125 extrusion-mold the tube 113 that is to accommodate the optical fibers 111 and the filler 115. The tube 113 is molded by injecting resin 127 through a circular flow path between the tip 123 and the die 125. In parallel, the optical fibers 111 and the gel 115 to be accommodated within the tube 113 are fed into the tube 113 through the needle 121.
In addition, in after-mentioned patent Documents 1 and 2, extrusion molding methods of a loose tube is disclosed. In these molding methods, an inner diameter of a die for bundling optical fibers in upstream of a needle is made smaller than that of the needle. In addition, gel is filled so as to function as thermal insulator between the optical fibers and the loose tube.
For example, describing the Patent Document 1, the optical fibers are being unbundled at an inlet of a cavity (portion filled with the gel), and fed into a spline member that bundles the optical fibers at an outlet of the cavity. On an outer circumferential surface of the spline member, slots are formed so as to flow the gel therethrough. Since the spline member is pressed onto a housing, it is presumed that an inner diameter of the housing is large enough not to oppilate the slots in order to flow the gel through. As described above, the bundled optical fibers are suffused around with the gel by making an opening diameter of the spline member (die) smaller than an inner diameter of the gel needle. As a result, heats affecting the optical fibers are relaxed by the gel layer while the optical fibers run through the needle in the extrusion head.
Patent Document 1: U.S. Pat. No. 5,395,557
Patent Document 1: U.S. Pat. No. 5,601,646 (Japanese Patent Application Laid-open No. H9-101442)